The present invention relates to methods and apparatus for logging earth formations penetrated by a borehole, and more particularly to methods and apparatus for compensating for borehole effects while logging such formations using nuclear means. While the invention will be particularly described with respect to methods and apparatus for compensated density logging of downhole earth formations (wherein gamma rays are emitted from a logging tool into the formation and Compton scattered back to the tool), the invention disclosure should be read with the understanding that it is equally applicable to borehole compensation of other nuclear logging measurements, such as natural gamma ray, neutron porosity, and so forth.
In recent years nuclear well logs measuring the density of downhole earth formations have become increasingly important to petroleum engineers and log analysts. Density logs are a standard means for determining formation porosity. When borehole conditions are reasonable, density readings can be combined with estimates of fluid and matrix densities to yield accurate porosity values. Measurements of the photoelectric factor (P.sub.e) have assisted lithology identification, and thereby improved the estimation of matrix density. However, previous P.sub.e measurements have often been adversely affected by rugose boreholes and/or the presence of barite in the drilling mud.
Porosities derived from density logs can be combined with measurements from resistivity or pulsed neutron logs to produce calculations of formation water saturations. Other combinations of density log data with that from sonic and/or neutron porosity logs can be used to determine formation lithology and to indicate formations with significant gas saturations. In certain areas even unsupplemented density log data can provide sufficient information to evaluate the formations of interest.
Density logging is based on the detection of attenuated gamma rays emitted from a radioactive source in a downhole tool. The gamma rays from the source penetrate through the toolcase, borehole, and formation. A fraction of these gamma rays are Compton scattered into and counted by one or more gamma ray detectors in the tool. The attenuation the gamma rays undergo between the source and detector(s) can, under certain conditions, be very simply related to formation bulk density. As a reasonable generalization, the count rate will decrease exponentially as the density of the formation/borehole system increases, and also as the source-detector spacing increases.
Gamma rays interact with matter principally by three processes: photoelectric absorption, pair production, and Compton scattering. Of these processes, only Compton scattering is not highly dependent upon the specific elements in the medium, depending instead upon only the density of the medium (the density being directly related to the number of electrons per unit volume). Photoelectric absorption and pair production, on the other hand, are strongly related to the atomic number Z of the nuclei in the formation and exhibit very strong gamma ray energy dependence. Therefore, current density logging tools are designed to respond primarily to Compton scattered gamma radiation, the selective response to such Compton scattered gamma radiation being achieved by proper selection of gamma ray energies and proper detector shielding.
Typically, present day density tools therefore measure density by observing an integrated Compton scattered gamma ray count rate over a broad, predetermined energy band. Since the higher density formation materials have higher gamma attenuation coefficients, the integrated count rate at the gamma ray detector in the logging tool will be lower when higher density material is present between the source and the detector. Using predetermined relationships, the count rates can then be converted into a measure of the formation density, and hence porosity.
In many tools, two detectors are provided and the data from both detectors are used simultaneously to provide borehole (especially mudcake) compensation. This is particularly true in compensated density logging, which uses a Cs.sup.137 source collimated into the formation and two collimated gamma ray detectors.
The conventional compensation concept is based upon the idea that when a borehole attenuating factor such as mudcake is present, the count rates at both detectors will be affected. Since mudcake occupies a larger percentage of the volume sensed by the short-spaced detector, the short-spaced count rate will change more than the long-spaced count rate. This will cause the short-spaced detector to indicate an apparent density which is different from that indicated by the long-spaced detector. The difference between these apparent densities is directly proportional to the amount of correction which needs to be applied to the long-spaced density to obtain the compensated density. Such compensation concepts and methods are well known. The concepts also rely on the fact that the density measurements are made by detecting higher energy scattered gamma rays which are more penetrating and are not isotropically distributed in the formation.
The addition of a photoelectric factor (P.sub.e) measurement to density logging has proved to be quite useful in lithology identification. However, because of limited depth of investigation and strong sensitivity to elements with large atomic number, P.sub.e measurements have been adversely affected by borehole rugosity, mudcake, washout, and barite in the drilling mud. A need therefore remains for a borehole logging method and apparatus which will give an accurate and repeatable determination of formation density, a reduction in borehole sensitivity for lithology determination, and a less borehole sensitive P.sub.e measurement.